Throughout history wood has been the natural fuel source; it is only in the last 200 years that fossil based fuels have taken over as the fuel of choice.
Although fossil based fuels have had a huge positive effect on our way of life, we are now experiencing the negative effect in the form of global warming.
I was once told in a rather philosophical way that there is a good reason why these fossil fuels are buried deep underground and while they served their purpose during the great industrial revolution in helping to advance our way of life, these fuels were never meant to be sustainable for all times.
Nature has provided us with many sustainable energy sources in the form of wind, sun, tidal currents, geothermal and wood, the challenge is harnessing and implementing this energy in the most efficient and environmentally sound way. With the effects of global warming more has to be done to reduce carbon dioxide (CO2) emissions and the only way forward is to change to more carbon friendly fuels and renewable technologies. The government under the Climate Change Act (2008) requires that by 2050, the UK’s annual CO2 emissions should be reduced by 80% compared to 1990 levels.
Currently home energy emissions are responsible for over a quarter of UK CO2 emissions. Also to be considered is that no renewable solution should be looked at in isolation, but considered as a part in the overall system for reducing CO2 emissions. Complementing technologies such as solar, wind, ground / air source heat pumps and biomass should be considered within the overall design of the heating / energy system.
One fuel that has seen resurgence as a carbon friendly fuel source is virgin wood, either in the form of traditional logs or biomass alternative such as pellets or chippings. Wood pellets are a renewable, clean burning biomass fuel made from recycled wood waste or sawdust, other options are wood chip, again from recycled waste. Wood is classed as a carbon neutral fuel on the basis that the amount of CO2 absorbed into the tree during its life is balanced by the amount of CO2 emitted when burnt or left to rot.
While open wood burning fires are estimated to be only 20-25% efficient, a modern wood burning stove can run at over 80% efficiency while contributing to the ambience of the room. Other advances in technology have lead to room sealed stoves which duct their combustion air from outside of the building, reducing air infiltration. Other technologies include biomass / pellet appliances which offer a very high efficiency and controllable output.
While all this talk of wood, efficiency and carbon reduction is great, the one thing often overlooked is the chimney. With the rapid introduction
of these new technologies, it is often found that standards, codes of practice, Building Regulations and even the appliance manufacture installation instructions fall short on the advice offered..
It is still widely accepted that solid biomass appliances are no different from any other solid fuel appliance with respect to the chimney design, sizing and height requirements.
It cannot be assumed that the internal diameter of the chimney will be the same as the outlet on the appliance. This will depend on the route and resistances within the chimney system while matching the combustion characteristics of the appliance. Further considerations such as appliance draught tolerances have to be factored into the chimney design together with the chimney performance over the output range of the appliance.
The design and sizing of the chimney system to the combustion characteristics of the appliance is critical for the correct performance and efficiency of the appliance and to ensure correct venting of the combustion products. To ensure correct sizing of the chimney, SFL use computer modelling (iSize) to determine the flow and thermal dynamics of the system to BS EN 13384-1, ensuring optimum performance of the system and economic product selection.
While the sizing and design of the system is critical, equal importance must be given to the actual chimney product employed for the application. Due to the efficiency and wide output range, the flue gas temperature will be much lower than traditional solid fuel appliances, in the region of 70°C-180°C. For this reason a well insulated chimney is essential for the rapid stabilisation of draught and to prevent the combustion product falling below their dew point.
Chimney heights are also another apparently grey area concerning biomass. Due to the potential for particulates in the flue gas, more Local Authorities are requiring detailed dispersal models to be undertaken as part of the planning requirement, usually using the AERMOD or ADMS 4 model.
The chimney product must also have a soot-fire resistance, should a chimney fire occur. The Nova chimney system, manufactured by SFL is CE Marked to BS EN 1856-1 T450 N1 W V2 L50050 G(50).
It is rated for flue gas temperatures up to 450 degrees C as standard, and conforms to the pressure/leakage requirement of N1 when tested to a positive pressure of 40Pa. Manufactured with a fully welded 0.5mm 1.4404 (316L) liner, independently tested to the corrosion class V2 wet and with a 25mm high density insulated annulus, Nova SM offers very high thermal and corrosion-resistant specification.
Soot-fire resistance is indicated by the G(50) classification indicating that the product has been tested at 1,000 degs C for 30 mins at a distance to combustible material of 50mm. With retrofitted seals, Nova SM has full condensate and pressure resistance up to 200Pa (P1), with the performance designation of BS EN 1856-1 T200 W P1 V2 L50050 O (50).
Technical Sales Manager John Lee.
